Simulation of cohesive particle flows in granular and gas-solid systems.

摘要

Cohesive interparticle forces may affect granular and fluidized flow applications by influencing minimum fluidization velocities and segregation behavior, and can lead to complete loss of flowability. These forces include van der Waals, liquid-briding, electrostatic, and magnetic forces. Despite the prevalence of cohesive effects in particulate flows, the incorporation of cohesion into continuum models is limited. Most cohesion models are defined as a continuous function of the separation distance between interacting particles. The continuous nature of these models conflicts with the assumption of instantaneous, binary collision inherent in the kinetic theory used to develop continuum models. A model that incorporates cohesive forces as binary, instantaneous impulses is the square-well model. In this work, the square-well model was incorporated into discrete-particle simulations of granular flow and fluidized flow to test the ability of the model to capture the physics of cohesive flows.;For simple shear flows, an investigation of the input parameter space indicates the presence of two distinct flow regimes. For large cohesive forces, a large, single agglomerate is formed. For moderate cohesive forces, the sheared system is composed of evenly distributed 2-particle, dynamic agglomerates. The results for this regime indicate that cohesion attenuates the stress components at higher solids fractions (in the collisional regime), as compared to the non-cohesive case. At lower solids fractions (kinetic regime), cohesive forces do not impact the observed stress.;Within a fluidized bed simulation, a method to map the parameters of the square-well model to equivalent parameters in the Hamaker model has been developed based on the minimum, relative normal velocity required to escape agglomeration in two-particle simulations. Mapping effectiveness was gauged by measuring the minimum fluidization velocity, mixing index, and average particle movement at varying levels of cohesion.;The cohesive fluidized bed simulation was used to study hysteresis behavior observed during defluidization-fluidization cycles of experimental fluidized beds. In both cohesion models, cohesive particle-particle are the primary cause for the pressure overshoot for the parameters considered. Simulations using the square-well cohesion model indicate that cohesive interactions between particles and the distributor plate (bottom wall) are a secondary mechanism whereas, simulations using the Hamaker model reveal that cohesive interactions between particles and the sidewalls are a secondary mechanism.